CN111566257B - Austenitic heat-resistant alloy, method for producing same, and austenitic heat-resistant alloy material - Google Patents

Austenitic heat-resistant alloy, method for producing same, and austenitic heat-resistant alloy material Download PDF

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CN111566257B
CN111566257B CN201980007668.XA CN201980007668A CN111566257B CN 111566257 B CN111566257 B CN 111566257B CN 201980007668 A CN201980007668 A CN 201980007668A CN 111566257 B CN111566257 B CN 111566257B
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resistant alloy
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austenitic heat
oxide
base material
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CN111566257A (en
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西本工
西山佳孝
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Abstract

The invention provides an austenitic heat-resistant alloy capable of obtaining sufficient corrosion resistance of molten salt even when exposed to molten salt at 600 ℃, a method for producing the same, and an austenitic heat-resistant alloy material. The austenitic heat-resistant alloy of the present application comprises a base material and Ni-Fe oxide having a spinel structure on the surface of the base material. The base material has the following chemical composition: c in mass%: 0.030 to 0.120 percent of Si: 0.02-1.00%, mn:0.10 to 2.00 percent of Cr:20.0% or more and less than 28.0%, ni: more than 35.0% and 50.0% or less, W:4.0 to 10.0 percent of Ti:0.01 to 0.30 percent of Nb:0.01 to 1.00 percent, sol.Al:0.0005 to 0.0400 percent, B:0.0005 to 0.0100%, mo: less than 0.5%, co:0 to 0.80 percent of Cu:0 to 0.50 percent and the balance: fe and impurities.

Description

Austenitic heat-resistant alloy, method for producing same, and austenitic heat-resistant alloy material
Technical Field
The present application relates to an austenitic heat-resistant alloy, a method for producing the same, and an austenitic heat-resistant alloy material.
Background
In recent years, a high-efficiency boiler has been developed for the purpose of energy saving. For example, ultra supercritical pressure boilers improve energy efficiency by making the temperature and pressure of the vapor higher than before. As a heat-resistant alloy tube for a high-efficiency boiler, for example, a seamless austenitic heat-resistant alloy tube described in japanese patent application laid-open No. 2013-104109 (patent document 1) has been proposed. In addition, boilers have been developed which use waste and biomass as fuels other than fossil fuels. Further, a power generation boiler using solar heat has been developed. In particular, a boiler for solar thermal power generation is attracting attention from the viewpoints of energy saving and environmental protection.
In general concentrating solar thermal power generation, which is solar thermal power generation, sunlight is concentrated and converted into heat. Steam is generated by using heat obtained by converting sunlight, and the turbine is rotated by the steam to generate electricity. The configuration of the concentrating solar thermal power generation system can be roughly classified into a concentrating/heat collecting device and a power generating device. The condensing/heat collecting devices currently used are parabolic-reflecting trough type, linear-fresnel type, tower type, disk type, etc.
A heat medium such as oil is used in a heat transfer tube of a conventional power generation boiler. However, with the recent increase in efficiency and temperature, molten salts such as molten nitrate, molten carbonate, molten sulfate and molten chloride salts are sometimes used as heat medium in concentrating/collecting devices for solar thermal power generation. In addition, the temperature inside the heat conduction pipe of the solar power collecting/condensing device increases to about 600 ℃. Therefore, heat-resistant steel used for heat pipes and the like of a light-collecting/heat-collecting device for solar thermal power generation is required to have high-temperature strength and corrosion resistance in high-temperature molten salt.
Japanese patent application laid-open publication No. 2013-199663 (patent document 2) proposes an austenitic stainless steel excellent in resistance to molten nitrate corrosion, characterized by C:0.1% or less, si:0.3% or more and 2.0% or less, mn: below 4.0%, ni:7% or more and 15% or less, cr:10% or more and 25% or less, mo:2.5% or less, cu:3.0% or less, V: less than 0.5%, N:0.3% or less, 0.5.ltoreq.Si+0.5 (Mo+Cu). Ltoreq.2.0%, the balance being Fe and unavoidable impurities, the ratio of elements other than oxygen constituting the oxide formed at the portion in contact with the molten nitrate at 600 ℃ or less satisfying Si+0.5 (Mo+Cu). Ltoreq.20% in atomic%. Patent document 2 describes that an austenitic stainless steel suitable for a portion in contact with molten nitrate in a temperature range of 400 to 600 ℃ can be obtained therefrom.
Japanese patent application laid-open No. 1-68449 (patent document 3) proposes a molten salt corrosion resistant material which is formed of an alloy containing Fe, cr and Ni, and the composition of Fe, cr and Ni shown as% by weight is referred to as C Fe 、C Cr And C Ni With k=c Fe ×C Cr +0.2×C Ni 2 The defined K value is in the range 1400-1800. Patent document 3 describes that a molten salt corrosion resistant material formed of an alloy that undergoes self-passivation, which spontaneously forms a film of Li-forming complex oxide excellent in corrosion resistance under the operating conditions of a molten carbonate fuel cell, can be provided.
Japanese patent application laid-open No. 8-41595 (patent document 4) proposes an Fe-Cr-Ni alloy steel having excellent corrosion resistance in a molten salt containing a chloride, characterized by containing, in weight%, C: less than 0.04%, si: less than 0.5%, mn: less than 1.5%, cr: more than 18% and less than 30%, ni: more than 10% and less than 35%, ca+mg:0.0005 to 0.005%, the ratio Cr/Fe of Cr content to Fe content is more than 0.33 and less than 0.7, and the ratio Ni/Fe of Ni content to Fe content is more than 0.33 and less than 1.0. Patent document 4 describes that an fe—cr—ni alloy steel that is inexpensive and has excellent corrosion resistance in a molten salt containing a chloride can be provided thereby.
Japanese patent laying-open No. 2016-50328 (patent document 5) proposes a tube member for a solar heat collecting tube, which is a tube member for a solar heat collecting tube for collecting solar heat to a heat medium flowing therein to heat the heat medium, and which is produced by a centrifugal casting method, wherein a high nickel layer is formed on an inner surface of a cylindrical body formed of a low nickel heat-resistant cast iron composed of a matrix formed of an Fe alloy containing 7 to 22 mass% of a basic element composed of carbon (C), silicon (Si), chromium (Cr), nickel (Ni), manganese (Mn) and copper (Cu), and the balance being iron (Fe), unavoidable impurities and a trace modification element of 1 mass% or less when the cast iron is 100 mass% as a whole, and wherein an austenite phase is a main phase in a normal temperature region. Patent document 5 describes that, in the case of using molten salt as a heat medium, it is thereby possible to provide a tube member for a solar heat collecting tube that can prevent corrosion even in a high-temperature state in which the temperature of the molten salt exceeds 600 ℃.
Prior art literature
Patent literature
Patent document 1: japanese patent application laid-open No. 2013-104109
Patent document 2: japanese patent laid-open publication No. 2013-199663
Patent document 3: japanese patent laid-open No. 1-68449
Patent document 4: japanese patent laid-open No. 8-41595
Patent document 5: japanese patent laid-open publication 2016-50328
Disclosure of Invention
Problems to be solved by the invention
However, even with the above-described technique, sufficient corrosion resistance (hereinafter referred to as molten salt corrosion resistance) may not be obtained when exposed to molten salt at 600 ℃.
The purpose of the present application is to provide an austenitic heat-resistant alloy which can obtain sufficient resistance to corrosion by molten salt even when exposed to molten salt at 600 ℃, a method for producing the austenitic heat-resistant alloy, and an austenitic heat-resistant alloy material.
Solution for solving the problem
The austenitic heat-resistant alloy of the present application comprises a base material and Ni-Fe oxide having a spinel structure on the surface of the base material. The base material has the following chemical composition: c in mass%: 0.030 to 0.120 percent of Si: 0.02-1.00%, mn:0.10 to 2.00 percent of Cr:20.0% or more and less than 28.0%, ni: more than 35.0% and 50.0% or less, W:4.0 to 10.0 percent of Ti:0.01 to 0.30 percent of Nb:0.01 to 1.00 percent, sol.Al:0.0005 to 0.0400 percent, B:0.0005 to 0.0100%, zr:0 to 0.1000 percent of Ca:0 to 0.0500 percent, REM:0 to 0.2000 percent of Hf:0 to 0.2000 percent, pd:0 to 0.2000 percent, P:0.040% or less, S: less than 0.010%, N: less than 0.020%, O: less than 0.0050%, mo: less than 0.5%, co:0 to 0.80 percent of Cu:0 to 0.50 percent and the balance: fe and impurities.
The austenitic heat-resistant alloy material of the present application comprises a base material and Cr 2 O 3 、(Fe、Cr、Ni) 3 O 4 And NaFeO 2 . The base material has the following chemical composition: c in mass%: 0.030 to 0.120 percent of Si: 0.02-1.00%, mn:0.10 to 2.00 percent of Cr:20.0% or more and less than 28.0%, ni: more than 35.0% and 50.0% or less, W:4.0 to 10.0 percent of Ti:0.01 to 0.30 percent of Nb:0.01 to 1.00 percent, sol.Al:0.0005 to 0.0400 percent, B:0.0005 to 0.0100%, zr:0 to 0.1000 percent of Ca:0 to 0.0500 percent, REM:0 to 0.2000 percent of Hf:0 to 0.2000 percent, pd:0 to 0.2000 percent, P:0.040% or less, S: less than 0.010%, N: less than 0.020%, O: less than 0.0050%, mo: less than 0.5%, co:0 to 0.80 percent of Cu:0 to 0.50 percent and the balance: fe and impurities. Cr (Cr) 2 O 3 Is disposed on the surface of the base material. (Fe, cr, ni) 3 O 4 Is arranged at Cr 2 O 3 And (3) upper part. NaFeO 2 Is arranged (Fe, cr, ni) 3 O 4 And (3) upper part.
The method for producing an austenitic heat-resistant alloy of the present application comprises a preparation step, a pretreatment step, an oxide scale removal step, and a Ni-Fe oxide formation step. In the preparation step, a billet having the chemical composition of the base metal of the austenitic heat-resistant alloy is prepared. In the pretreatment step, the blank is immersed in a solution containing nitric acid and hydrofluoric acid, and subjected to pretreatment. In the scale removal step, the ingot is taken out of the solution, and the scale on the surface of the ingot is removed. In the Ni-Fe oxide forming step, the ingot from which the oxide scale on the surface has been removed is immersed in a solution containing nitric acid and hydrofluoric acid, the concentration of nitric acid being higher than that of hydrofluoric acid, and Ni-Fe oxide having a spinel structure is formed on the surface of the ingot.
ADVANTAGEOUS EFFECTS OF INVENTION
The austenitic heat-resistant alloy and austenitic heat-resistant alloy material of the present application have sufficient resistance to molten salt corrosion even when exposed to molten salt at 600 ℃. The austenitic heat-resistant alloy of the present application is obtained, for example, by the production method of the present application.
Detailed Description
The present inventors have conducted studies to improve the resistance to molten salt corrosion of austenitic heat-resistant alloys and austenitic heat-resistant alloy materials. As a result, the following findings were obtained.
Conventionally, cr has been formed on the surface of heat-resistant steel to improve the corrosion resistance of heat-resistant steel 2 O 3 A Cr oxide film as a main body. This suppresses the outward diffusion of the components of the heat-resistant steel, and improves the corrosion resistance of the heat-resistant steel. However, cr 2 O 3 Is active for melting molten salt such as nitrate. Thus, cr 2 O 3 Dissolution occurs in the molten salt in the form of chromate ions. Therefore, in the conventional method of forming a Cr oxide film, it is difficult to improve the resistance to molten salt corrosion of the austenitic heat-resistant alloy.
On the other hand, when a coating mainly composed of Fe oxide is formed on the surface of the base material of the austenitic heat-resistant alloy, for example, fe is formed on the surface of the base material of the austenitic heat-resistant alloy in the molten salt 3 O 4 And Fe (Fe) 2 O 3 As a main body of scale. Fe (Fe) 3 O 4 And Fe (Fe) 2 O 3 The growth rate of (2) is obviously fast. Further, fe 3 O 4 And Fe (Fe) 2 O 3 Since oxygen in-diffusion from molten salt cannot be suppressed, it is difficult to improve the resistance to corrosion by molten salt of austenitic heat-resistant alloy by a film mainly composed of Fe oxide.
On the other hand, when a coating mainly composed of Ni oxide is formed on the surface of the base material, for example, niO is formed on the surface of the base material of the austenitic heat-resistant alloy in molten salt. The oxidation rate of NiO is high. Further, niO cannot suppress the in-diffusion of oxygen, na, K, and the like from molten salts, and therefore, even in the case of a coating film mainly composed of Ni oxide, it is difficult to improve the resistance to corrosion by molten salts of heat-resistant steel.
The inventors of the present invention have found as a result of intensive studies that: the molten salt corrosion resistance of the austenitic heat-resistant alloy can be improved by forming an oxide different from the conventional Cr oxide film on the surface of the base material of the austenitic heat-resistant alloy.
Specifically, in the examples described below, molten salt corrosion tests were performed using the alloy materials shown in Table 1, and corrosion reduction (mg/cm 2 ). As a result of the molten salt corrosion test, as shown in Table 2, the corrosion loss of the alloy sheet of test No. 4 was 3.6mg/cm 2 The corrosion loss of the alloy sheet below test No. 13 was 10.8mg/cm 2 . However, the chemical composition of the base material of the alloy plate of test No. 4 was the same as that of the base material of the alloy plate of test No. 13.
The inventors of the present invention analyzed the oxides on the surface of the base material of the alloy sheets of test No. 4 and test No. 13 in detail for the purpose of explaining the cause thereof.
The inventors of the present invention first analyzed the cross section of the oxide on the surface of the base material of the alloy sheet of test No. 4 by XRD. The depth profile of the oxide on the surface of the base material of the alloy sheet of test No. 4 is as follows: the surface oxide of the base material of the alloy sheet of test No. 4 contains Ni and Fe. Next, the inventors performed raman analysis on the oxide on the surface of the base material of the alloy plate of test No. 4. As a result, it was confirmed that 700 to 710cm unique to the spinel-structured oxide was found in the oxide on the surface of the base material of the alloy sheet of test No. 4 -1 Is a peak of (2).
On the other hand, cr-based alloy sheet was formed on the surface of the base material of the alloy sheet of test No. 13 2 O 3 Conventional Cr oxide films as a main body.
The present inventors considered from the above analysis results that: if the austenitic heat-resistant alloy has a spinel-structured Ni-Fe oxide on the surface of the base material, the resistance to molten salt corrosion is improved. The reason why the spinel-structured ni—fe oxide improves the resistance to molten salt corrosion of the austenitic heat-resistant alloy is considered as follows. When an austenitic heat-resistant alloy includes a Ni-Fe oxide having a spinel structure on the surface of a base material, naFeO having the Ni-Fe oxide as a core and having a high growth rate is formed in a molten salt in the initial period of corrosion 2 。NaFeO 2 Unlike the conventional Cr oxide film, it is difficult to dissolve in molten saltAnd (5) solving. Thus, naFeO 2 Contact between the molten salt and the base metal of the austenitic heat-resistant alloy is suppressed. Thereafter, in NaFeO 2 Is formed between the alloy and a base metal of an austenitic heat-resistant alloy (Fe, cr, ni) 3 O 4 。(Fe、Cr、Ni) 3 O 4 The internal diffusion of molten salt components (Na ions, K ions) into the base metal side of the austenitic heat-resistant alloy is suppressed. Thereafter, the alloy is further described in (Fe, cr, ni) 3 O 4 Cr oxide is formed between the alloy and a base metal of the austenitic heat-resistant alloy. Cr oxide suppresses the outward diffusion of the components of the austenitic heat-resistant alloy. Thereby, the growth of the scale is suppressed. As a result, corrosion of the base metal is suppressed, and the resistance to molten salt corrosion of the austenitic heat-resistant alloy is improved.
The austenitic heat-resistant alloy of the present application, which has been completed based on the above-described findings, includes a base material and ni—fe oxide having a spinel structure on the surface of the base material. The base material has the following chemical composition: c in mass%: 0.030 to 0.120 percent of Si: 0.02-1.00%, mn:0.10 to 2.00 percent of Cr:20.0% or more and less than 28.0%, ni: more than 35.0% and 50.0% or less, W:4.0 to 10.0 percent of Ti:0.01 to 0.30 percent of Nb:0.01 to 1.00 percent, sol.Al:0.0005 to 0.0400 percent, B:0.0005 to 0.0100%, zr:0 to 0.1000 percent of Ca:0 to 0.0500 percent, REM:0 to 0.2000 percent of Hf:0 to 0.2000 percent, pd:0 to 0.2000 percent, P:0.040% or less, S: less than 0.010%, N: less than 0.020%, O: less than 0.0050%, mo: less than 0.5%, co:0 to 0.80 percent of Cu:0 to 0.50 percent and the balance: fe and impurities.
The austenitic heat-resistant alloy of the present application includes a Ni-Fe oxide having a spinel structure on the surface of a base material. Therefore, the austenitic heat-resistant alloy of the present application has sufficient resistance to corrosion by molten salt even when exposed to molten salt at 600 ℃.
The above Ni-Fe oxide having a spinel structure preferably contains NiFe 2 O 4 。
The austenitic heat-resistant alloy preferably further includes Cr oxide between the base material and the ni—fe oxide.
In this case, the resistance to molten salt corrosion of the austenitic heat-resistant alloy is further improved.
The Cr oxide preferably contains Cr selected from 2 O 3 And Cr (V) 2 O 3 ·yH 2 1 or 2 of the group consisting of O. Here, y is an arbitrary rational number.
The austenitic heat-resistant alloy preferably contains Zr in mass% of the chemical composition of the base material: 0.0005 to 0.1000 percent.
In this case, the high-temperature strength of the austenitic heat-resistant alloy is improved.
The austenitic heat-resistant alloy preferably contains, in mass%, ca: 0.0005-0.0500%.
In this case, the hot workability of the austenitic heat-resistant alloy is improved.
The austenitic heat-resistant alloy preferably has a chemical composition, in mass%, of a base material selected from REM:0.0005 to 0.2000 percent of Hf:0.0005 to 0.2000% and Pd:0.0005 to 0.2000% of at least 1 member of the group consisting of.
In this case, the creep strength of the austenitic heat-resistant alloy is improved.
The austenitic heat-resistant alloy material of the present application comprises a base material and Cr 2 O 3 、(Fe、Cr、Ni) 3 O 4 And NaFeO 2 . The base material has the following chemical composition: c in mass%: 0.030 to 0.120 percent of Si: 0.02-1.00%, mn:0.10 to 2.00 percent of Cr:20.0% or more and less than 28.0%, ni: more than 35.0% and 50.0% or less, W:4.0 to 10.0 percent of Ti:0.01 to 0.30 percent of Nb:0.01 to 1.00 percent, sol.Al:0.0005 to 0.0400 percent, B:0.0005 to 0.0100%, zr:0 to 0.1000 percent of Ca:0 to 0.0500 percent, REM:0 to 0.2000 percent of Hf:0 to 0.2000 percent, pd:0 to 0.2000 percent, P:0.040% or less, S: less than 0.010%, N: less than 0.020%, O: less than 0.0050%, mo: less than 0.5%, co:0 to 0.80 percent of Cu:0 to 0.50 percent and the balance: fe and impurities. Cr (Cr) 2 O 3 Is disposed on the surface of the base material. (Fe, cr, ni) 3 O 4 Is arranged at Cr 2 O 3 And (3) upper part. NaFeO 2 Is arranged (Fe, cr, ni) 3 O 4 And (3) upper part.
The austenitic heat-resistant alloy material of the present application comprises Cr on the surface of a base material in order from the base material side 2 O 3 、(Fe、Cr、Ni) 3 O 4 And NaFeO 2 . Therefore, the austenitic heat-resistant alloy material of the present application exhibits excellent resistance to molten salt corrosion.
The base material of the austenitic heat-resistant alloy material preferably contains Zr in mass%: 0.0005 to 0.1000 percent.
In this case, the high-temperature strength of the austenitic heat-resistant alloy material is improved.
The base material of the austenitic heat-resistant alloy material preferably contains, in mass%, ca: 0.0005-0.0500%.
In this case, the hot workability of the austenitic heat-resistant alloy material is improved.
The base material of the austenitic heat-resistant alloy material preferably has a chemical composition, in mass%, selected from REM:0.0005 to 0.2000 percent of Hf:0.0005 to 0.2000% and Pd:0.0005 to 0.2000% of at least 1 member of the group consisting of.
In this case, the creep strength of the austenitic heat-resistant alloy material is improved.
The method for producing an austenitic heat-resistant alloy of the present application comprises a preparation step, a pretreatment step, an oxide scale removal step, and a Ni-Fe oxide formation step. In the preparation step, a billet having the chemical composition of the base metal of the austenitic heat-resistant alloy is prepared. In the pretreatment step, the blank is immersed in a solution containing nitric acid and hydrofluoric acid, and subjected to pretreatment. In the scale removal step, the ingot is taken out of the solution, and the scale on the surface of the ingot is removed. In the Ni-Fe oxide forming step, the ingot from which the oxide scale on the surface has been removed is immersed in a solution containing nitric acid and hydrofluoric acid, the concentration of nitric acid being higher than that of hydrofluoric acid, and Ni-Fe oxide having a spinel structure is formed on the surface of the ingot.
The austenitic heat-resistant alloy of the present application will be described in detail below.
[ Austenitic Heat-resistant alloy ]
The austenitic heat-resistant alloy of the present application comprises a base material and Ni-Fe oxide having a spinel structure on the surface of the base material.
[ chemical composition of base Material ]
The chemical composition of the base material of the austenitic heat-resistant alloy of the present application contains the following elements. Unless otherwise specified, the term "% related to an element" means mass%.
C：0.030～0.120％
Carbon (C) forms carbide, and is an element necessary for obtaining high-temperature tensile strength and high-temperature creep strength required as an austenitic heat-resistant alloy for high temperatures of about 600 ℃. If the C content is too low, this effect is not obtained. On the other hand, if the C content is too high, undissolved carbides are generated. If the C content is too high, cr carbide is excessively formed, and the weldability of the austenitic heat-resistant alloy is lowered. Therefore, the C content is 0.030 to 0.120%. The lower limit of the C content is preferably 0.040%, more preferably 0.050%. The upper limit of the C content is preferably 0.110%, more preferably 0.100%.
Si：0.02～1.00％
Silicon (Si) is also an element necessary for improving the oxidation resistance of austenitic heat-resistant alloys when added as a deoxidizer in steelmaking. If the Si content is too low, this effect is not obtained. On the other hand, if the Si content is too high, the hot workability of the austenitic heat-resistant alloy is lowered. Therefore, the Si content is 0.02 to 1.00%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%. The upper limit of the Si content is preferably 0.80%, more preferably 0.50%.
Mn：0.10～2.00％
Manganese (Mn) bonds with the impurity S contained in the austenitic heat-resistant alloy to form MnS, and improves hot workability. If the Mn content is too low, this effect is not obtained. On the other hand, if the Mn content is too high, embrittlement of the austenitic heat-resistant alloy occurs, and the hot workability is rather deteriorated. If the Mn content is too high, the weldability of the austenitic heat-resistant alloy is also lowered. Therefore, the Mn content is 0.10 to 2.00%. The lower limit of the Mn content is preferably 0.20%, more preferably 0.30%, and still more preferably 0.50%. The upper limit of the Mn content is preferably 1.80%, more preferably 1.50%, and still more preferably 1.20%.
Cr:20.0% or more and less than 28.0%
Chromium (Cr) is an important element for improving the corrosion resistance of molten salts. Cr also improves the oxidation resistance of the austenitic heat-resistant alloy. In order to ensure excellent molten salt corrosion resistance in molten salt at 400 to 600 ℃, a Cr content of 20.0% or more is required. It has been conventionally thought that the corrosion resistance increases as the Cr content increases. However, if the Cr content is too high, a Cr oxide film mainly composed of Cr oxide is formed. Since the Cr oxide film is dissolved in the molten salt, the resistance to corrosion by molten salt of the austenitic heat-resistant alloy is lowered. If the Cr content is too high, the structural stability is also lowered, and the creep strength of the austenitic heat-resistant alloy is lowered. If the Cr content is too high, the weldability of the austenitic heat-resistant alloy is lowered. Therefore, the Cr content is 20.0% or more and less than 28.0%. The lower limit of the Cr content is preferably 20.5%, more preferably 21.0%, and still more preferably 22.0%. The upper limit of the Cr content is preferably 27.5%, more preferably 26.5%, and still more preferably 26.0%.
Ni: more than 35.0% and 50.0% or less
Nickel (Ni) is an element that stabilizes an austenite structure, and is also an important alloying element for ensuring molten salt corrosion resistance. In order to obtain a stable austenite structure, ni needs to be more than 35.0% from the balance with the Cr content described above. On the other hand, when the Ni content is too high, a single-layer oxide film of NiO is formed in the molten salt. At this time, the resistance to corrosion by molten salt of the austenitic heat-resistant alloy in the molten salt is lowered. If the Ni content is too high, an increase in cost is also incurred. If the Ni content is too high, the creep strength of the austenitic heat-resistant alloy is further lowered. Therefore, the Ni content exceeds 35.0% and is 50.0% or less. The lower limit of the Ni content is preferably 38.5%, more preferably 40.0%, and even more preferably 41.0%. The upper limit of the Ni content is preferably 48.0%, more preferably 47.0%, and even more preferably 45.0%.
W：4.0～10.0％
Tungsten (W) suppresses grain boundary sliding creep that occurs preferentially in a high temperature region by solid solution strengthening. If the W content is too low, this effect is not obtained. On the other hand, if the W content is too high, the austenitic heat-resistant alloy is excessively hardened, and therefore, the hot workability of the austenitic heat-resistant alloy is lowered. If the W content is too high, the weldability of the austenitic heat-resistant alloy is also lowered. Therefore, the W content is 4.0 to 10.0%. The lower limit of the W content is preferably 4.5%, more preferably 6.0%. The upper limit of the W content is preferably 9.0%, more preferably 8.0%.
Ti：0.01～0.30％
Titanium (Ti) forms carbonitrides and precipitates, and the high-temperature strength of the austenitic heat-resistant alloy is improved. If the Ti content is too low, this effect is not obtained. On the other hand, if the Ti content is too high, undissolved carbonitrides and/or oxides are formed, facilitating the grain mixing of the austenite grains. If the Ti content is too high, uneven creep deformation and a decrease in ductility are also caused. Therefore, the Ti content is 0.01 to 0.30%. The lower limit of the Ti content is preferably 0.03%, more preferably 0.05%. The upper limit of the Ti content is preferably 0.25%, more preferably 0.20%.
Nb：0.01～1.00％
Niobium (Nb) forms carbonitrides and precipitates, and the high-temperature strength of the austenitic heat-resistant alloy is improved. If the Nb content is too low, this effect is not obtained. On the other hand, if the Nb content is too high, the weldability of the austenitic heat-resistant alloy decreases. Therefore, the Nb content is 0.01 to 1.00%. The lower limit of the Nb content is preferably 0.05%, more preferably 0.10%. The upper limit of the Nb content is preferably 0.60%, more preferably 0.50%.
sol.Al：0.0005～0.0400％
Aluminum (Al) is used as a deoxidizer. If the Al content is too low, this effect is not obtained. On the other hand, if Al remains in a large amount, the structural stability of the austenitic heat-resistant alloy decreases. Therefore, the Al content is 0.0005 to 0.0400%. The lower limit of the Al content is preferably 0.0010%, more preferably 0.0050%. The upper limit of the Al content is preferably 0.0300%, more preferably 0.0200%. In the present application, the Al content refers to the content of acid-soluble Al (sol.al).
B：0.0005～0.0100％
Boron (B) reduces the contents of N and O described later, and suppresses oxides and nitrides. If the B content is too low, this effect is not obtained. On the other hand, if the B content is too high, the weldability of the austenitic heat-resistant alloy decreases. Therefore, the B content is 0.0005 to 0.0100%. The lower limit of the B content is preferably 0.0007%, more preferably 0.0010%. The upper limit of the B content is preferably 0.0080%, more preferably 0.0050%.
The balance of the chemical composition of the base material of the austenitic heat-resistant alloy of the present application is Fe and impurities. Here, the impurities in the chemical composition of the base material means: in the industrial production of an austenitic heat-resistant alloy, substances mixed from ores, scraps, production environments, and the like as raw materials are allowed within a range that does not adversely affect the austenitic heat-resistant alloy of the present application.
[ about optional elements ]
The chemical composition of the base material of the austenitic heat-resistant alloy of the present application may contain the following elements as optional elements.
Zr：0～0.1000％
Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When included, zr strengthens grain boundaries to improve the high-temperature strength of the austenitic heat-resistant alloy. This effect can be obtained by slightly containing Zr. On the other hand, if the Zr content is too high, oxides and nitrides not dissolved in solid are formed similarly to Ti, and grain boundary sliding creep and uneven creep deformation are promoted. If the Zr content is too high, creep strength and ductility in a high temperature region of the austenitic heat-resistant alloy are also reduced. Accordingly, the Zr content is 0 to 0.1000%. The lower limit of the Zr content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the Zr content is preferably 0.0600%.
Ca：0～0.0500％
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, ca bonds to S to stabilize S, and the hot workability of the austenitic heat-resistant alloy is improved. This effect can be obtained if Ca is contained slightly. On the other hand, if the Ca content is too high, the toughness, ductility, and steel quality of the austenitic heat-resistant alloy decrease. Therefore, the Ca content is 0 to 0.0500%. The lower limit of the Ca content is preferably 0.0005%. The upper limit of the Ca content is preferably 0.0100%.
REM：0～0.2000％
Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. In the case of inclusion, REM forms stable oxides and sulfides, suppressing the undesirable effects of O and S. When REM is contained, the austenitic heat-resistant alloy is improved in corrosion resistance, hot workability, creep strength and creep ductility. This effect can be obtained by slightly containing REM. On the other hand, if the REM content is too high, inclusions such as oxides are excessively formed, and the hot workability and weldability of the austenitic heat-resistant alloy are lowered. Therefore, REM content is 0 to 0.2000%. The lower limit of the REM content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the REM content is preferably 0.1000%. In the present application, REM means 17 elements from lanthanum (La) of element number 57 to lutetium (Lu) of element number 71 in the periodic table, plus yttrium (Y) and scandium (Sc). REM content refers to the total content of these elements.
Hf：0～0.2000％
Hafnium (Hf) is an optional element and may not be included. That is, the Hf content may be 0%. In the case of inclusion, hf forms stable oxides, sulfides, suppressing the undesirable effects of O and S. When Hf is contained, the austenitic heat-resistant alloy is improved in corrosion resistance, hot workability, creep strength and creep ductility. This effect can be obtained as long as Hf is contained slightly. On the other hand, if the Hf content is too high, inclusions such as oxides are excessively formed, and the hot workability and weldability of the austenitic heat-resistant alloy are lowered. Therefore, the Hf content is 0 to 0.2000%. The lower limit of the Hf content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the Hf content is preferably 0.1000%.
Pd：0～0.2000％
Palladium (Pd) is an optional element and may not be present. That is, the Pd content may be 0%. In the case of inclusion, pd forms stable oxides, sulfides, suppressing the undesirable effects of O and S. When Pd is contained, the austenitic heat-resistant alloy is improved in corrosion resistance, hot workability, creep strength and creep ductility. This effect can be obtained if Pd is contained slightly. On the other hand, if the Pd content is too high, inclusions such as oxides are excessively formed, and the hot workability and weldability of the austenitic heat-resistant alloy are lowered. Therefore, the Pd content is 0 to 0.2000%. The lower limit of the Pd content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the Pd content is preferably 0.1000%.
The impurities include, for example, the following elements. The content of these elements is limited for the following reasons.
P: less than 0.040 percent
Phosphorus (P) is an impurity inevitably contained. That is, the lower limit of the P content exceeds 0%. P reduces weldability and hot workability of the austenitic heat-resistant alloy. Therefore, the P content is 0.040% or less. The preferable upper limit of the P content is 0.030%. The lower the P content, the more preferred. However, an extreme reduction in the P content greatly increases the manufacturing cost. Therefore, the preferable lower limit of the P content is 0.0005% in consideration of industrial production.
S: less than 0.010%
Sulfur (S) is an impurity inevitably contained. That is, the lower limit of the S content exceeds 0%. S reduces weldability and hot workability of the austenitic heat-resistant alloy. Therefore, the S content is 0.010% or less. The preferable upper limit of the S content is 0.008%. The lower the S content, the more preferred. However, when a certain amount of S is contained to improve the fluidity of molten steel during welding, 0.004% or more may be contained.
N: less than 0.020%
Nitrogen (N) is an impurity which is inevitably contained. That is, the lower limit of the N content exceeds 0%. If the N content is too high, undissolved carbonitrides of Ti and B are formed, and the structure of the austenitic heat-resistant alloy forms mixed grains. In this case, grain boundary sliding creep and uneven creep deformation in a high temperature region are facilitated, and the strength of the austenitic heat-resistant alloy is lowered. Therefore, the N content is less than 0.020%. The upper limit of the N content is preferably 0.016%, more preferably 0.010%. The lower the N content, the more preferred. However, an extreme reduction in the N content greatly increases manufacturing costs. Therefore, in consideration of industrial production, the preferable lower limit of the N content is 0.005%.
O: less than 0.0050%
Oxygen (O) is an impurity which is inevitably contained. That is, the lower limit of the O content exceeds 0%. If the O content is too high, undissolved oxides of Ti and Al are formed, and the structure of the austenitic heat-resistant alloy forms mixed grains. In this case, grain boundary sliding creep and uneven creep deformation in a high temperature region are facilitated, and the strength of the austenitic heat-resistant alloy is lowered. Therefore, the O content is 0.0050% or less. The upper limit of the O content is preferably 0.0030%. The lower the O content, the more preferred. However, an extreme reduction in the O content would greatly increase the manufacturing cost. Therefore, the preferable lower limit of the O content is 0.0005% in consideration of industrial production.
Mo: less than 0.5%
Molybdenum (Mo) is an impurity inevitably contained. That is, the lower limit of the Mo content exceeds 0%. If the Mo content is too high, an embrittlement layer is formed in the austenitic heat-resistant alloy under a high-temperature environment. If the Mo content is too high, the corrosion resistance of the austenitic heat-resistant alloy is also lowered. Therefore, the Mo content is less than 0.5%. The upper limit of the Mo content is preferably 0.3%, more preferably 0.1%. The lower the Mo content, the more preferred. However, an extreme decrease in Mo content greatly increases manufacturing costs. Therefore, the preferable lower limit of the Mo content is 0.01% in consideration of industrial production.
Co：0～0.80％
Cobalt (Co) is an impurity sometimes mixed from scraps or the like. There are cases where Co is not contained. That is, the Co content may be 0%. If the Co content is too high, the hot workability of the austenitic heat-resistant alloy is lowered. Therefore, co is not actively added. The Co content is 0-0.80%. When Co is contained, the lower limit of Co content exceeds 0%. However, when a certain amount of Co is contained to improve creep strength, 0.01% or more may be contained.
Cu：0～0.50％
Copper (Cu) is an impurity sometimes mixed from scraps or the like. There are cases where Cu is not contained. That is, the Cu content may be 0%. If the Cu content is too high, grain boundary sliding creep in a high temperature region is facilitated to be long. Therefore, cu is not positively added. Cu content is 0-0.50%. When Cu is contained, the lower limit of Cu content exceeds 0%. The upper limit of the Cu content is preferably 0.20%. However, when Cu is contained in a certain amount to improve strength, 0.01% or more may be contained.
[ microstructure of base Material and shape of Austenitic Heat-resistant alloy ]
The microstructure of the base material of the austenitic heat-resistant alloy of the present application is an austenitic single phase after precipitation is removed. The shape of the austenitic heat-resistant alloy of the present application is not particularly limited. The austenitic heat-resistant alloy may be in the shape of a tube, a plate, a rod, a wire, or a section steel. Austenitic heat-resistant alloys may be preferably used as the tube.
[ Ni-Fe oxide ]
The austenitic heat-resistant alloy includes a Ni-Fe oxide having a spinel structure on the surface of a base material. Thus, naFeO having Ni-Fe oxide as a core and having a high growth rate is formed in the molten salt in the initial stage of corrosion 2 。NaFeO 2 Unlike the conventional Cr oxide film, it is difficult to dissolve in molten salt. Thus, naFeO 2 Contact of the molten salt with the base metal of the heat-resistant steel is suppressed. Thereafter, a layer (Fe, cr, ni) 3 O 4 And Cr oxide, the resistance to corrosion by molten salt of the austenitic heat-resistant alloy is improved.
The ni—fe oxide having a spinel-type structure refers to an oxide having a spinel-type structure and containing Ni and Fe. Spinel-type structures belonging to the cubic system and usable as AB 2 X 4 A structure of an inorganic compound represented by the formula (I). In the case of Ni-Fe oxide, A or B in the composition formula corresponds to Ni or Fe, and X in the composition formula corresponds to O. The Ni-Fe oxide having spinel structure may be NiFe 2 O 4 Ni may also be 2 FeO 4 . The ni—fe oxide having a spinel-type structure is optionally substituted with Cr in part. The Ni-Fe oxide having a spinel-type structure may be, for example (Ni, fe, cr) 3 O 4 . That is, ni-Fe oxide having spinel structure is selected from NiFe 2 O 4 、Ni 2 FeO 4 And (Ni, fe, cr) 3 O 4 1 or more than 2 kinds of the group consisting of. The Ni-Fe oxide having a spinel structure preferably contains NiFe 2 O 4 。
The ni—fe oxide having a spinel structure may cover the entire surface of the base material of the austenitic heat-resistant alloy or may cover a part of the surface of the base material. The ni—fe oxide having a spinel structure may be formed in contact with the surface of the base material of the austenitic heat-resistant alloy or may be formed without contact. The ni—fe oxide having a spinel structure may be the outermost layer or may not be the outermost layer. If Ni-Fe oxide is disposed on the surface of the base material of the austenitic heat-resistant alloy, naFeO is generated in the molten salt 2 And thereafter (Fe, cr, ni) 3 O 4 And Cr (V) 2 O 3 The resistance to molten salt corrosion of the austenitic heat-resistant alloy is improved. On the other hand, in the case of forming Ni-Fe oxide having spinel-type structure in the form of the outermost layer, naFeO 2 Is promoted by NaFeO 2 The effect of suppressing contact between the molten salt and the base metal of the austenitic heat-resistant alloy is improved. Therefore, the ni—fe oxide having a spinel-type structure is preferably formed in the form of the outermost layer.
[ method for identifying Ni-Fe oxide ]
The Ni-Fe oxide having a spinel-type structure was identified by the following method. First, a test piece containing an oxide on the surface of a base material of an austenitic heat-resistant alloy was taken. The surface of the oxide was subjected to XPS (X-ray Photoelectron Spectroscopy) to prepare a depth profile along the thickness direction of the oxide. The elements obtained by the depth profile are subjected to a state analysis, and are separated into the concentration of each element in the form of an oxide and the concentration of each element in the form of a metal. It was confirmed that Ni and Fe were contained in the element existing in the form of oxide in a range from the surface of the oxide to a depth position when the detection intensity of O (oxygen) reached half the detection intensity of oxygen at the surface of the oxide. XPS was measured using the following conditions.
Device: XPS measuring device (ULVAC-PHI company, quantura SXM)
X-ray: mono-alkα ray (hν= 1486.6 eV), X-ray diameter: 100 μm phi
Neutralization gun: 1.0V, 20. Mu.A
Sputtering conditions: ar (Ar) + Acceleration voltage: 1kV and grating: 2X 2mm
Sputtering speed: 1.8 nm/min (SiO) 2 Converted value
Next, raman spectroscopy analysis was performed on the oxide on the surface of the base material of the austenitic heat-resistant alloy. From the spectrum obtained by Raman spectrum analysis, 700-710 cm unique to oxide of spinel structure was identified -1 Is a peak of (2). Thus, it was confirmed that Ni-Fe oxide was formed on the surface of the base material of the austenitic heat-resistant alloy. Raman spectroscopy was performed using the following conditions.
Device: microscopic laser Raman spectrum measuring device manufactured by horiba of Kagaku Co., ltd. (LabRAMHR Evolution)
Measurement configuration: 180 ° backscatter configuration
Excitation wavelength: 532nm
Diffraction grating score: 600 strips/mm
ND filter: 25 percent of
Power: 2.3mW
Objective lens: 50 times of
[ thickness of Ni-Fe oxide ]
The thickness of the Ni-Fe oxide is preferably, for example, 2.0 to 7.0nm. When the Ni-Fe oxide has a thickness of 2.0nm or more, cr formation on the surface of the austenitic heat-resistant alloy is suppressed when the alloy is used in a molten salt environment 2 O 3 Promote NaFeO 2 Therefore, the resistance to molten salt corrosion of the austenitic heat-resistant alloy is stably improved. On the other hand, the upper limit of the thickness of the Ni-Fe oxide is not particularly limited. However, if the thickness of the Ni-Fe oxide is 7.0nm or less, the treatment time can be suppressed from becoming extremely long, and it is preferable from the viewpoint of manufacturability. The lower limit of the thickness of the Ni-Fe oxide is more preferably 4.0nm. The upper limit of the thickness of the Ni-Fe oxide is more preferably 6.0nm.
[ method for measuring thickness of Ni-Fe oxide ]
As in the case of the above-described ni—fe oxide identification method, XPS measurement was performed on the oxide on the surface of the base material of the austenitic heat-resistant alloy in the thickness direction of the oxide. Points 2 before and after the peak position at which the concentration (at%) of Ni and Fe in the form of oxide reached the maximum were respectively designated as points a and B. The depth-wise distance between the A-point and the B-point is defined as the thickness of the Ni-Fe oxide.
[ Cr oxide ]
The austenitic heat-resistant alloy preferably further includes Cr oxide between the base material of the austenitic heat-resistant alloy and the ni—fe oxide. In molten salt, cr oxide forms Cr 2 O 3 The outward diffusion of the base material component of the austenitic heat-resistant alloy is suppressed. Thus, the growth of scale is inhibited. In addition, if Cr oxide is formed between the base material of the austenitic heat-resistant alloy and the Ni-Fe oxide, cr is contained in the molten salt 2 O 3 Is arranged in NaFeO 2 Below (base material of austenitic heat-resistant alloy and NaFeO) 2 Between). At this time, naFeO is used 2 To inhibit Cr 2 O 3 Contact with molten salt. Thus, cr 2 O 3 Dissolution in molten salt is inhibited. That is, in the case of a two-layer structure in which Cr oxide and ni—fe oxide are laminated in this order from the surface of the base material of the austenitic heat-resistant alloy, the resistance to corrosion by molten salt of the austenitic heat-resistant alloy is further improved.
The Cr oxide is an oxide of Cr. The Cr oxide is selected from, for example, cr 2 O 3 、CrO·OH·xH 2 O (hydrated oxyhydroxide) and Cr 2 O 3 ·yH 2 O (hydrous oxide) 1 or 2 or more of the group consisting of O. The Cr oxide preferably contains a material selected from Cr 2 O 3 And Cr (V) 2 O 3 ·yH 2 1 or 2 of the group consisting of O. Here, crO.OH.xH 2 X in O is an arbitrary rational number. x is usually about 1 to 2. Cr (Cr) 2 O 3 ·yH 2 Y in O is an arbitrary rational number. y is usually about 1 to 2.
[ composition of Cr oxide and method for identifying the position of Cr oxide ]
Under the same conditions as in the above-described method for identifying ni—fe oxide, a depth profile was produced by XPS for a test piece containing an oxide on the surface of a base material of an austenitic heat-resistant alloy. The elements obtained by the depth profile are separated into the concentration of each element in the form of oxide and the concentration of each element in the form of metal by performing a state analysis. The inclusion of Cr in the element present as an oxide was confirmed within a range from the surface of the oxide to a depth position at which the detection intensity of O (oxygen) reached half the detection intensity of oxygen at the surface of the oxide. Further, it was confirmed from the depth profile that the peaks of Ni and Fe in the oxide form were present at a position shallower than the peak of Cr in the oxide form (a position close to the surface of the oxide). Thus, it was confirmed that Cr oxide exists between the base material of the austenitic heat-resistant alloy and the Ni-Fe oxide.
In order to identify the composition of the Cr oxide, raman spectrum analysis was performed on the oxide on the surface of the base material of the austenitic heat-resistant alloy under the same conditions as those of the above-described ni—fe oxide identification method, based on XPS analysis. Identification of Cr from a spectrogram obtained by Raman Spectroscopy 2 O 3 550cm peculiar to -1 Nearby peaks or Cr 2 O 3 ·yH 2 850cm peculiar to O -1 A nearby peak. Thus, the composition of Cr oxide was identified.
[ thickness of Cr oxide ]
The thickness of the Cr oxide is, for example, 2.0 to 10.0nm. When the Cr oxide has a thickness of 2.0nm or more, the coating is accelerated in NaFeO when used in a molten salt environment 2 Cr is formed between the alloy and an austenitic heat-resistant alloy 2 O 3 The resistance to molten salt corrosion of austenitic heat-resistant alloys is further stably improved. On the other hand, if the Cr oxide has a thickness of 10.0nm or less, naFeO is more easily formed in a molten salt environment 2 . The lower limit of the thickness of the Cr oxide is preferably 3.0nm. The upper limit of the thickness of the Cr oxide is preferably 8.0nm.
[ method for measuring thickness of Cr oxide ]
As in the case of the Cr oxide identification method, XPS measurement was performed on the oxide on the surface of the base material of the austenitic heat-resistant alloy along the thickness direction of the oxide. Points 2 at which the concentration (at%) of Cr in the form of oxide reached half the maximum value of the Cr concentration (at%) before and after the peak position reached the maximum were designated as points C and D, respectively. The depth-wise distance between points C-D is defined as the thickness of the Cr oxide.
[ method of production ]
The austenitic heat-resistant alloy of the present application can be produced by, for example, the following production method. The manufacturing method comprises a preparation step, a pretreatment step, an oxide scale removal step and a Ni-Fe oxide formation step. Hereinafter, a method for manufacturing a seamless steel pipe will be described as an example. However, the manufacturing method of the present application is not limited to the case of manufacturing a seamless steel pipe.
[ preparation procedure ]
In the preparation step, a billet having the chemical composition of the base metal of the austenitic heat-resistant alloy is prepared. The billet may be a slab, a square billet, or a bar-shaped billet manufactured by a continuous casting method including round billet continuous casting. Further, the steel ingot produced by the ingot casting method may be a bar-shaped billet produced by hot working. It is also possible to use a bar-shaped blank produced from a slab or square blank by hot working.
And loading the blank into a heating furnace or a soaking furnace for heating. The heating temperature is, for example, 1100 to 1350 ℃. The heated blank is thermally processed. For example, as the hot working, mannessman method is performed. Specifically, the billet is pierced by a piercing mill to produce a tube blank. Then, the billet is drawn and rolled by a mandrel mill and a sizing mill to produce a seamless steel pipe. As the hot working, hot extrusion may be performed, or hot forging may be performed. The temperature of the thermal processing is, for example, 500 to 1100 ℃.
The blank produced by the hot working may be subjected to a heat treatment or cold working, as necessary. Cold working is for example cold rolling or cold drawing. When cold working is performed, heat treatment may be performed to control the structure of the blank. After the heat treatment, the surface may be descaled (oxide scale formed on the surface is removed by shot blasting, pickling, or the like). Finally, cleaning is carried out, so that foreign matters on the surface can be removed. Through the above steps, a blank material as a seamless steel pipe is produced.
The blank may also be a steel plate. At this time, the blank is hot worked to produce a steel sheet. Further, the steel plate may be welded to manufacture a welded steel pipe.
After hot working (after cold working in the case of cold working) and before the pretreatment step, the blank may be immersed in sulfuric acid. Thus, ni-Fe oxide is more easily formed.
[ pretreatment Process ]
In the pretreatment step, the blank is immersed in a pretreatment solution containing nitric acid and hydrofluoric acid, and subjected to pretreatment. Thus, cr is formed on the surface of the steel material after hot working or cold working 2 O 3 、Fe 3 O 4 The scale dissolves and floats from the surface of the billet. The scale is removed by a subsequent scale removal process and subjected to a ni—fe oxide formation process, whereby a ni—fe oxide can be formed. The concentration of nitric acid in the pretreatment solution is, for example, 5 to 15 mass%. The concentration of hydrofluoric acid in the pretreatment solution is, for example, 2 to 5 mass%. The pretreatment solution further contains a solvent in addition to nitric acid and hydrofluoric acid. The solvent means 1 or 2 selected from the group consisting of, for example, water and an organic solvent dispersed or dissolved in water. The pretreatment solution may contain other ingredients. The other components are, for example, 1 or 2 or more selected from the group consisting of ions of metal elements contained in the chemical composition of Fe ions, cr ions, ni ions, W ions, mo ions, and other base materials, and surfactants. The total content of other components may be 5% by mass or less. Among other components, the contents of Fe ion and Ni ion are particularly limited. The Fe ion content is 3.8 mass% or less, and the Ni ion content is 0.7 mass% or less. When the Fe ion or Ni ion is contained in an amount exceeding the above content, the Ni-Fe oxygen described later In the oxide forming step, formation of the ni—fe oxide having a spinel structure is hindered. On the other hand, if Fe ions and Ni ions are excessively reduced, the productivity is reduced, and therefore, 0.9 mass% or more of Fe ions and 0.1 mass% or more of Ni ions are allowed to be contained.
The temperature of the pretreatment solution (treatment temperature) in the pretreatment step and the time for immersing the blank in the pretreatment solution (treatment time) can be appropriately set. The treatment temperature is, for example, 20 to 50 ℃. The treatment time is, for example, 2 to 25 hours.
[ procedure for removing oxide skin ]
In the scale removal step, the ingot is taken out of the pretreatment solution, and the scale on the surface of the ingot is removed. The method of removing the scale is, for example, water washing, shot blasting, or the like. By performing the scale removal step, the scale on the surface is removed, and the ni—fe oxide can be formed in the subsequent ni—fe oxide formation step.
[ step of Forming Ni-Fe oxide ]
In the Ni-Fe oxide formation step, the ingot from which the surface oxide scale has been removed is immersed in a Ni-Fe oxide formation solution, and a Ni-Fe oxide having a spinel structure is formed on the surface of the ingot. The Ni-Fe oxide forming solution contains nitric acid and hydrofluoric acid, the concentration of the nitric acid being higher than the concentration of the hydrofluoric acid. By making the concentration of nitric acid higher than that of hydrofluoric acid, the formation of Ni-Fe oxide is promoted. The concentration of the nitric acid in the ni—fe oxide forming solution is, for example, 5 to 15 mass%. The concentration of hydrofluoric acid in the ni—fe oxide forming solution is, for example, 2 to 5 mass%. The concentration of nitric acid in the Ni-Fe oxide forming solution is preferably higher than the concentration of nitric acid in the pretreatment solution. The concentration of hydrofluoric acid in the ni—fe oxide forming solution is preferably higher than the concentration of hydrofluoric acid in the pretreatment solution.
The Ni-Fe oxide forming solution further contains a solvent in addition to nitric acid and hydrofluoric acid. The solvent means 1 or 2 selected from the group consisting of, for example, water and an organic solvent dispersed or dissolved in water. The Ni-Fe oxide forming solution may contain other additives. The other additives are 1 or 2 or more selected from the group consisting of ions of metal elements contained in the chemical composition of, for example, fe ion, cr ion, ni ion, W ion, mo ion, and other base materials, and surfactants. The total content of other additives may be 3.5 mass% or less. Among other components, the contents of Fe ion and Ni ion are particularly limited. The Fe ion content is 1.2 mass% or less and the Ni ion content is 0.3 mass% or less. When the Fe ion or the Ni ion is contained in an amount exceeding the above content, the formation of Ni-Fe oxide having a spinel structure is hindered. On the other hand, if Fe ions and Ni ions are excessively reduced, the productivity is reduced, and therefore, 0.05 mass% or more of Fe ions and 0.1 mass% or more of Ni ions are allowed to be contained.
The temperature of the ni—fe oxide forming solution (treatment temperature) and the time for immersing the ingot in the ni—fe oxide forming solution (treatment time) in the ni—fe oxide forming step can be appropriately set. The treatment temperature is, for example, 20 to 50 ℃. The treatment time is, for example, 2 to 25 hours.
For example, the austenitic heat-resistant alloy of the present application can be produced by the above steps.
The austenitic heat-resistant alloy material of the present application will be described in detail below.
[ Austenitic Heat-resistant alloy Material ]
The austenitic heat-resistant alloy material of the present application comprises a base material and Cr 2 O 3 、(Fe、Cr、Ni) 3 O 4 And NaFeO 2 。
[ chemical composition of base Material ]
The chemical composition of the base material of the austenitic heat-resistant alloy material of the present application contains the following elements. Unless otherwise specified, the term "% related to an element" means mass%.
C：0.030～0.120％
Carbon (C) forms carbide, and is an element necessary for obtaining high-temperature tensile strength and high-temperature creep strength required as an austenitic heat-resistant alloy material for high temperatures of about 600 ℃. If the C content is too low, this effect is not obtained. On the other hand, if the C content is too high, undissolved carbides are generated. If the C content is too high, cr carbide is excessively formed, and the weldability of the austenitic heat-resistant alloy material is lowered. Therefore, the C content is 0.030 to 0.120%. The lower limit of the C content is preferably 0.040%, more preferably 0.050%. The upper limit of the C content is preferably 0.110%, more preferably 0.100%.
Si：0.02～1.00％
Silicon (Si) is added as a deoxidizer in steelmaking, and is also an element necessary for improving the oxidation resistance of austenitic heat-resistant alloy materials. If the Si content is too low, this effect is not obtained. On the other hand, if the Si content is too high, the hot workability of the austenitic heat-resistant alloy material is lowered. Therefore, the Si content is 0.02 to 1.00%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%. The upper limit of the Si content is preferably 0.80%, more preferably 0.50%.
Mn：0.10～2.00％
Manganese (Mn) bonds with the impurity S contained in the austenitic heat-resistant alloy material to form MnS, thereby improving hot workability. If the Mn content is too low, this effect is not obtained. On the other hand, if the Mn content is too high, embrittlement of the austenitic heat-resistant alloy material occurs, and the hot workability is rather deteriorated. If the Mn content is too high, the weldability of the austenitic heat-resistant alloy material is also lowered. Therefore, the Mn content is 0.10 to 2.00%. The lower limit of the Mn content is preferably 0.20%, more preferably 0.30%, and still more preferably 0.50%. The upper limit of the Mn content is preferably 1.80%, more preferably 1.50%, and still more preferably 1.20%.
Cr:20.0% or more and less than 28.0%
Chromium (Cr) is an important element for improving the corrosion resistance of molten salts. Cr further improves the oxidation resistance of the austenitic heat-resistant alloy material. In order to ensure excellent molten salt corrosion resistance in molten salt at 400 to 600 ℃, a Cr content of 20.0% or more is required. It has been conventionally thought that the corrosion resistance increases as the Cr content increases. However, if the Cr content is too high, a Cr oxide film mainly composed of Cr oxide is formed. Since the Cr oxide film is dissolved in the molten salt, the resistance to corrosion by molten salt of the austenitic heat-resistant alloy material is lowered. If the Cr content is too high, the structural stability is also lowered, and the creep strength of the austenitic heat-resistant alloy material is lowered. If the Cr content is too high, the weldability of the austenitic heat-resistant alloy material is lowered. Therefore, the Cr content is 20.0% or more and less than 28.0%. The lower limit of the Cr content is preferably 20.5%, more preferably 21.0%, and still more preferably 22.0%. The upper limit of the Cr content is preferably 27.5%, more preferably 26.5%, and still more preferably 26.0%.
Ni: more than 35.0% and 50.0% or less
Nickel (Ni) is an element that stabilizes an austenite structure, and is also an important alloying element for ensuring molten salt corrosion resistance. In order to obtain a stable austenite structure, ni needs to be more than 35.0% from the balance with the Cr content described above. On the other hand, when the Ni content is too high, the molten salt will be rich in (Fe, cr, ni) 3 O 4 NiO is formed thereon. At this time, the resistance to corrosion by molten salt of the austenitic heat-resistant alloy material in molten salt is lowered. If the Ni content is too high, an increase in cost is also incurred. If the Ni content is too high, the creep strength of the austenitic heat-resistant alloy material is further lowered. Therefore, the Ni content exceeds 35.0% and is 50.0% or less. The lower limit of the Ni content is preferably 38.5%, more preferably 40.0%, and even more preferably 41.0%. The upper limit of the Ni content is preferably 48.0%, more preferably 47.0%, and even more preferably 45.0%.
W：4.0～10.0％
Tungsten (W) suppresses grain boundary sliding creep that occurs preferentially in a high temperature region by solid solution strengthening. If the W content is too low, this effect is not obtained. On the other hand, if the W content is too high, the austenitic heat-resistant alloy material is excessively hardened, and therefore, the hot workability of the austenitic heat-resistant alloy material is lowered. If the W content is too high, the weldability of the austenitic heat-resistant alloy material is also lowered. Therefore, the W content is 4.0 to 10.0%. The lower limit of the W content is preferably 4.5%, more preferably 6.0%. The upper limit of the W content is preferably 9.0%, more preferably 8.0%.
Ti：0.01～0.30％
Titanium (Ti) forms carbonitrides and precipitates, and the high-temperature strength of the austenitic heat-resistant alloy material is improved. If the Ti content is too low, this effect is not obtained. On the other hand, if the Ti content is too high, undissolved carbonitrides and/or oxides are formed, facilitating the grain mixing of the austenite grains. If the Ti content is too high, uneven creep deformation and a decrease in ductility are also caused. Therefore, the Ti content is 0.01 to 0.30%. The lower limit of the Ti content is preferably 0.03%, more preferably 0.05%. The upper limit of the Ti content is preferably 0.25%, more preferably 0.20%.
Nb：0.01～1.00％
Niobium (Nb) forms carbonitrides and precipitates, and the high-temperature strength of the austenitic heat-resistant alloy material is improved. If the Nb content is too low, this effect is not obtained. On the other hand, if the Nb content is too high, the weldability of the austenitic heat-resistant alloy material decreases. Therefore, the Nb content is 0.01 to 1.00%. The lower limit of the Nb content is preferably 0.05%, more preferably 0.10%. The upper limit of the Nb content is preferably 0.60%, more preferably 0.50%.
sol.Al：0.0005～0.0400％
Aluminum (Al) is used as a deoxidizer. If the Al content is too low, this effect is not obtained. On the other hand, if Al remains in a large amount, the structural stability of the austenitic heat-resistant alloy material decreases. Therefore, the Al content is 0.0005 to 0.0400%. The lower limit of the Al content is preferably 0.0010%, more preferably 0.0050%. The upper limit of the Al content is preferably 0.0300%, more preferably 0.0200%. In the present application, the Al content refers to the content of acid-soluble Al (sol.al).
B：0.0005～0.0100％
Boron (B) reduces the contents of N and O described later, and suppresses oxides and nitrides. If the B content is too low, this effect is not obtained. On the other hand, if the B content is too high, the weldability of the austenitic heat-resistant alloy material decreases. Therefore, the B content is 0.0005 to 0.0100%. The lower limit of the B content is preferably 0.0007%, more preferably 0.0010%. The upper limit of the B content is preferably 0.0080%, more preferably 0.0050%.
The balance of the chemical composition of the base material of the austenitic heat-resistant alloy material of the present application is Fe and impurities. Here, the impurities in the chemical composition of the base material means: in the industrial production of the austenitic heat-resistant alloy material, substances mixed from ores, scraps, production environments, and the like as raw materials are allowed within a range that does not adversely affect the austenitic heat-resistant alloy material of the present application.
[ about optional elements ]
The chemical composition of the base material of the austenitic heat-resistant alloy material of the present application may contain the following elements as optional elements.
Zr：0～0.1000％
Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When included, zr strengthens grain boundaries to improve the high-temperature strength of the austenitic heat-resistant alloy material. This effect can be obtained by slightly containing Zr. On the other hand, if the Zr content is too high, oxides and nitrides not dissolved in solid are formed similarly to Ti, and grain boundary sliding creep and uneven creep deformation are promoted. If the Zr content is too high, creep strength and ductility in a high temperature region of the austenitic heat-resistant alloy material also decrease. Accordingly, the Zr content is 0 to 0.1000%. The lower limit of the Zr content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the Zr content is preferably 0.0600%.
Ca：0～0.0500％
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, ca bonds to S to stabilize S, and the hot workability of the austenitic heat-resistant alloy material is improved. This effect can be obtained if Ca is contained slightly. On the other hand, if the Ca content is too high, the toughness, ductility, and steel quality of the austenitic heat-resistant alloy material decrease. Therefore, the Ca content is 0 to 0.0500%. The lower limit of the Ca content is preferably 0.0005%. The upper limit of the Ca content is preferably 0.0100%.
REM：0～0.2000％
Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. In the case of inclusion, REM forms stable oxides and sulfides, suppressing the undesirable effects of O and S. When REM is contained, the austenitic heat-resistant alloy material is improved in corrosion resistance, hot workability, creep strength and creep ductility. This effect can be obtained by slightly containing REM. On the other hand, if the REM content is too high, inclusions such as oxides are excessively formed, and the hot workability and weldability of the austenitic heat-resistant alloy material are lowered. Therefore, REM content is 0 to 0.2000%. The lower limit of the REM content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the REM content is preferably 0.1000%. In the present application, REM means 17 elements from lanthanum (La) of element number 57 to lutetium (Lu) of element number 71 in the periodic table, plus yttrium (Y) and scandium (Sc). REM content refers to the total content of these elements.
Hf：0～0.2000％
Hafnium (Hf) is an optional element and may not be included. That is, the Hf content may be 0%. In the case of inclusion, hf forms stable oxides, sulfides, suppressing the undesirable effects of O and S. When Hf is contained, the austenitic heat-resistant alloy material is improved in corrosion resistance, hot workability, creep strength and creep ductility. This effect can be obtained as long as Hf is contained slightly. On the other hand, if the Hf content is too high, inclusions such as oxides are excessively formed, and the hot workability and weldability of the austenitic heat-resistant alloy material are lowered. Therefore, the Hf content is 0 to 0.2000%. The lower limit of the Hf content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the Hf content is preferably 0.1000%.
Pd：0～0.2000％
Palladium (Pd) is an optional element and may not be present. That is, the Pd content may be 0%. In the case of inclusion, pd forms stable oxides, sulfides, suppressing the undesirable effects of O and S. When Pd is contained, the austenitic heat-resistant alloy material is improved in corrosion resistance, hot workability, creep strength, and creep ductility. This effect can be obtained if Pd is contained slightly. On the other hand, if the Pd content is too high, inclusions such as oxides are excessively formed, and the hot workability and weldability of the austenitic heat-resistant alloy material are lowered. Therefore, the Pd content is 0 to 0.2000%. The lower limit of the Pd content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the Pd content is preferably 0.1000%.
The impurities include, for example, the following elements. The content of these elements is limited for the following reasons.
P: less than 0.040 percent
Phosphorus (P) is an impurity inevitably contained. That is, the lower limit of the P content exceeds 0%. P reduces weldability and hot workability of the austenitic heat-resistant alloy material. Therefore, the P content is 0.040% or less. The preferable upper limit of the P content is 0.030%. The lower the P content, the more preferred. However, an extreme reduction in the P content greatly increases the manufacturing cost. Therefore, the preferable lower limit of the P content is 0.0005% in consideration of industrial production.
S: less than 0.010%
Sulfur (S) is an impurity inevitably contained. That is, the lower limit of the S content exceeds 0%. S reduces weldability and hot workability of the austenitic heat-resistant alloy material. Therefore, the S content is 0.010% or less. The preferable upper limit of the S content is 0.008%. The lower the S content, the more preferred. However, when a certain amount of S is contained to improve the fluidity of molten steel during welding, 0.004% or more may be contained.
N: less than 0.020%
Nitrogen (N) is an impurity which is inevitably contained. That is, the lower limit of the N content exceeds 0%. If the N content is too high, undissolved carbonitrides of Ti and B are formed, and the structure of the austenitic heat-resistant alloy material forms mixed grains. In this case, the grain boundary sliding creep and the uneven creep deformation in the high temperature region are facilitated, and the strength of the austenitic heat-resistant alloy material is lowered. Therefore, the N content is less than 0.020%. The upper limit of the N content is preferably 0.016%, more preferably 0.010%. The lower the N content, the more preferred. However, an extreme reduction in the N content greatly increases manufacturing costs. Therefore, in consideration of industrial production, the preferable lower limit of the N content is 0.005%.
O: less than 0.0050%
Oxygen (O) is an impurity which is inevitably contained. That is, the lower limit of the O content exceeds 0%. If the O content is too high, undissolved oxides of Ti and Al are formed, and the structure of the austenitic heat-resistant alloy material forms mixed grains. In this case, the grain boundary sliding creep and the uneven creep deformation in the high temperature region are facilitated, and the strength of the austenitic heat-resistant alloy material is lowered. Therefore, the O content is 0.0050% or less. The upper limit of the O content is preferably 0.0030%. The lower the O content, the more preferred. However, an extreme reduction in the O content would greatly increase the manufacturing cost. Therefore, the preferable lower limit of the O content is 0.0005% in consideration of industrial production.
Mo: less than 0.5%
Molybdenum (Mo) is an impurity inevitably contained. That is, the lower limit of the Mo content exceeds 0%. If the Mo content is too high, an embrittlement layer is generated in the austenitic heat-resistant alloy material under a high-temperature environment. If the Mo content is too high, the corrosion resistance of the austenitic heat-resistant alloy material also decreases. Therefore, the Mo content is less than 0.5%. The upper limit of the Mo content is preferably 0.3%, more preferably 0.1%. The lower the Mo content, the more preferred. However, an extreme decrease in Mo content greatly increases manufacturing costs. Therefore, the preferable lower limit of the Mo content is 0.01% in consideration of industrial production.
Co：0～0.80％
Cobalt (Co) is an impurity sometimes mixed from scraps or the like. There are cases where Co is not contained. That is, the Co content may be 0%. If the Co content is too high, the hot workability of the austenitic heat-resistant alloy material is lowered. Therefore, co is not actively added. The Co content is 0-0.80%. When Co is contained, the lower limit of Co content exceeds 0%. However, when a certain amount of Co is contained to improve creep strength, 0.01% or more may be contained.
Cu：0～0.50％
Copper (Cu) is an impurity sometimes mixed from scraps or the like. There are cases where Cu is not contained. That is, the Cu content may be 0%. If the Cu content is too high, grain boundary sliding creep in a high temperature region is facilitated to be long. Therefore, cu is not positively added. Cu content is 0-0.50%. When Cu is contained, the lower limit of Cu content exceeds 0%. The upper limit of the Cu content is preferably 0.20%. However, when Cu is contained in a certain amount to improve strength, 0.01% or more may be contained.
[ microstructure of base Material and shape of Austenitic Heat-resistant alloy Material ]
The microstructure of the base material of the austenitic heat-resistant alloy material of the present application is an austenitic single phase after removing precipitates. The shape of the austenitic heat-resistant alloy material of the present application is not particularly limited. The austenitic heat-resistant alloy material may be in the shape of a pipe, a plate, a rod, a wire, or a section steel. An austenitic heat-resistant alloy material can be suitably used as the tube.
The austenitic heat-resistant alloy material further comprises Cr 2 O 3 、(Fe、Cr、Ni) 3 O 4 And NaFeO 2 。Cr 2 O 3 Is disposed on the surface of a base material of an austenitic heat-resistant alloy material. (Fe, cr, ni) 3 O 4 Is arranged at Cr 2 O 3 And (3) upper part. NaFeO 2 Is arranged (Fe, cr, ni) 3 O 4 And (3) upper part. In other words, these oxides are formed on the surface of the base material of the austenitic heat-resistant alloy material in the order of Cr from the surface side of the base material 2 O 3 、(Fe、Cr、Ni) 3 O 4 、NaFeO 2 Is a sequential stack of layers.
[Cr 2 O 3 ]
Cr 2 O 3 Is formed on the surface of a base material of an austenitic heat-resistant alloy material. Cr (Cr) 2 O 3 The outward diffusion of the base material component of the austenitic heat-resistant alloy material is suppressed. Thus, the formation of scale is suppressed. Furthermore, cr 2 O 3 Base material formed on austenitic heat-resistant alloy material and NaFeO 2 Between them. NaFeO 2 Is not dissolved in molten salt, inhibits molten salt and Cr 2 O 3 Is a contact of (a) with a substrate. Thus, cr 2 O 3 Dissolution in molten salt is inhibited. Due to Cr 2 O 3 Is inhibited and thus its effect is maintained. As a result, the resistance to molten salt corrosion of the austenitic heat-resistant alloy material is improved.
[Cr 2 O 3 Is a method of identification of (2)]
Cr on surface of base material of austenitic heat-resistant alloy material 2 O 3 The identification was performed by the following method. The austenitic heat-resistant alloy material was cut along the thickness direction of the oxide on the surface of the austenitic heat-resistant alloy material, and a sample containing the oxide was taken. For the section of the oxide of the sample, XRD (X-ray diffraction) and EPMA (electric on Probe Micro Analyzer) are analyzed.
XRD was measured using the following conditions.
Device: RINT-2500 manufactured by Physics Co
X-ray vacuum tube: co ray
Wavelength of incident light:
Figure BDA0002574815450000291
scan range: 2θ=10 to 102°
Scanning step size: 0.02 degree
EPMA was measured under the following conditions.
Device: electron beam microscopic analyzer (JXA-8530F manufactured by Japanese electronics company)
Measurement magnification: 5000 times
Acceleration voltage: 15.0kV
Measurement method: element mapping
Measurement range: 18 μm in the thickness direction of the oxide x 18 μm in the direction perpendicular to the thickness of the oxide
As a result of analysis by XRD, cr was confirmed 2 O 3 Is a peak of (2). Further, it was confirmed by the analysis of Cr mapping by EPMA that a region having a higher Cr concentration than that of the base material of the austenitic heat-resistant alloy material exists in the oxide. Thus, it was found that Cr was formed on the surface of the base material of the austenitic heat-resistant alloy material 2 O 3 。
[Cr 2 O 3 Thickness of (2)]
Cr 2 O 3 For example, 0.3 to 3.5 μm. If Cr 2 O 3 When the thickness of (2) is 0.3 μm or more, the outward diffusion of alloy components from the alloy and the inward diffusion of oxygen from the molten salt are stably suppressed, and the molten salt corrosion resistance of the austenitic heat-resistant alloy material is further stably improved. Cr (Cr) 2 O 3 The lower limit of the thickness of (2) is preferably 0.5. Mu.m. The upper limit of the thickness of the Cr oxide is preferably 2.0. Mu.m.
[Cr 2 O 3 Method for measuring thickness of (2)]
Cr 2 O 3 The thickness of (2) was measured by the following method. The austenitic heat-resistant alloy material was cut along the thickness direction of the oxide on the surface of the base material of the austenitic heat-resistant alloy material, and test pieces were taken. For the section of the oxide, the above Cr is used 2 O 3 Under the same conditions as the identification method of (a), an EPMA-based elemental mapping analysis was performed. The Cr concentration based on the element map is divided into 3 stages in the range where oxygen (O) is detected by the element map. The total area of the region (Cr-enriched layer) having the highest Cr concentration among the Cr concentrations in 3 stages was calculated. The total area of the Cr-rich layer obtained was divided by the length of the measurement range in the direction perpendicular to the thickness of the oxide. The obtained value was defined as Cr 2 O 3 Is a thickness of (c).
[(Fe、Cr、Ni) 3 O 4 ]
(Fe、Cr、Ni) 3 O 4 Is arranged at Cr 2 O 3 And (3) upper part. (Fe, cr, ni) 3 O 4 The internal diffusion of molten salt components (Na ions, K ions) into the base material side of the austenitic heat-resistant alloy material is suppressed. Therefore, the resistance to molten salt corrosion of the austenitic heat-resistant alloy material is improved.
[(Fe、Cr、Ni) 3 O 4 Is a method of identification of (2)]
(Fe、Cr、Ni) 3 O 4 The identification was performed by the following method. The austenitic heat-resistant alloy material was cut along the thickness direction of the oxide on the surface of the austenitic heat-resistant alloy material, and a sample containing the oxide was taken. For the section of the oxide of the sample, the above Cr is used 2 O 3 The same conditions as the identification method of (a) were used for analysis using XRD and EPMA. As a result of the analysis, peaks of spinel phases (Fe-Cr spinel, ni-Cr spinel, fe-Ni spinel and Fe-Cr-Ni spinel) were confirmed by XRD. Then, in the range where oxygen (O) was detected by the element mapping, it was confirmed that the detection regions of Fe, cr, and Ni were overlapped. Cr described above 2 O 3 Neutralization of NaFeO described later 2 The Ni concentration in (C) is low. For Cr in 2 O 3 (Cr-enriched layer) and NaFeO 2 Between and Ni concentration ratioCr 2 O 3 (Cr-enriched layer) neutralization of NaFeO 2 The medium-high region (Ni-enriched layer) was determined. Thus, it was identified as (Fe, cr, ni) 3 O 4 Further, it was identified that (Fe, cr, ni) 3 O 4 Is arranged at Cr 2 O 3 And (3) upper part.
[(Fe、Cr、Ni) 3 O 4 Thickness of (2)]
(Fe、Cr、Ni) 3 O 4 For example, 0.5 to 5.0 μm. If (Fe, cr, ni) 3 O 4 When the thickness of (2) is 0.5 μm or more, the inward diffusion of Na ions, K ions, etc. from the molten salt is stably suppressed, and therefore, the molten salt corrosion resistance of the austenitic heat-resistant alloy material is more stably improved. (Fe, cr, ni) 3 O 4 The lower limit of the thickness of (2) is preferably 1.0. Mu.m. (Fe, cr, ni) 3 O 4 The upper limit of the thickness of (2) is preferably 3.0. Mu.m.
[(Fe、Cr、Ni) 3 O 4 Method for measuring thickness of (2)]
With Cr as described above 2 O 3 In the same manner as in the identification method (a), EPMA analysis (element mapping) was performed on the cross section of the oxide on the surface of the base material of the austenitic heat-resistant alloy material. To the extent oxygen (O) is detected by elemental mapping, the oxygen is to be found in Cr 2 O 3 (Cr-enriched layer) and NaFeO 2 And Ni concentration ratio Cr between 2 O 3 (Cr-enriched layer) neutralization of NaFeO 2 The medium-high region (Ni-enriched layer) was determined. The total area of the Ni-enriched layers in the measurement range was calculated. The total area of the obtained Ni-enriched layer was divided by the length of the measurement range in the direction perpendicular to the thickness of the oxide. The values obtained are defined as (Fe, cr, ni) 3 O 4 Is a thickness of (c).
[NaFeO 2 ]
NaFeO 2 Formed at (Fe, cr, ni) 3 O 4 And (3) upper part. NaFeO 2 Is difficult to dissolve in molten salt. Thus, naFeO 2 Inhibiting formation of Cr thereunder 2 O 3 Contact with molten salt. NaFeO 2 Further, contact between the base material of the austenitic heat-resistant alloy material and the molten salt is suppressed. Therefore, the refractory salt of the austenitic heat-resistant alloy materialThe corrosiveness is improved.
[NaFeO 2 Is a method of identification of (2)]
By using Cr as described above 2 O 3 Under the same conditions as the identification method, the surface oxide cross section of the base material of the austenitic heat-resistant alloy material was analyzed by XRD and EPMA. Based on the analysis results of XRD, naFeO was confirmed 2 Is a peak of (2). Next, in the range where oxygen (O) was detected by element mapping, it was confirmed that the detection regions of Fe and Na were overlapped. Thus, it was identified as NaFeO 2 . Further, the depth of the detection region of Na (the position of the oxide in the depth direction) was confirmed. Thus, it was identified that the composition was in (Fe, cr, ni) 3 O 4 On which NaFeO is arranged 2 。
[NaFeO 2 Thickness of (2)]
NaFeO 2 For example, 0.5 to 7.0 μm. If NaFeO 2 When the thickness of (2) is 0.5 μm or more, the contact between the molten salt and the base metal of the austenitic heat-resistant alloy material is stably blocked, and therefore, the resistance to corrosion by molten salt of the austenitic heat-resistant alloy material is more stably improved. NaFeO 2 The lower limit of the thickness of (2) is preferably 1.0. Mu.m. NaFeO 2 The upper limit of the thickness of (2) is preferably 5.0. Mu.m.
[NaFeO 2 Method for measuring thickness of (2)]
With Cr as described above 2 O 3 In the same manner as in the identification method of (a), EPMA analysis (element mapping) was performed on the cross section of the oxide on the surface of the base material of the austenitic heat-resistant alloy material. The area where Na is detected (Na-enriched layer) is determined within the range where oxygen (O) is detected by element mapping. The total area of the Na-enriched layer in the measurement range was calculated. The total area of the obtained Na-rich layer was divided by the length of the measurement range in the direction perpendicular to the thickness of the oxide. The obtained value was defined as NaFeO 2 Is a thickness of (c).
[ method of production ]
An example of a method for producing the austenitic heat-resistant alloy material of the present application will be described. The austenitic heat-resistant alloy material can be produced, for example, by bringing an austenitic heat-resistant alloy having a Ni-Fe oxide having a spinel structure on the surface of the base material into contact with or immersing the alloy in a molten salt at 500 ℃ or higher. The upper temperature limit of the molten salt is, for example, 800 ℃. Molten salt means 1 or 2 or more selected from the group consisting of, for example, molten nitrate, molten carbonate, molten sulfate and molten chloride salt.
Nitrate is a salt with nitrate ions as anions. The nitrate is preferably a salt of a nitrate ion with 1 or 2 or more selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions. The nitrate refers to 1 or 2 or more selected from the group consisting of, for example, lithium nitrate, sodium nitrate, potassium nitrate, calcium nitrate, ammonium nitrate, magnesium nitrate, and barium nitrate.
Carbonate is a salt having carbonate ions as anions. The carbonate is preferably a salt of a carbonate ion with 1 or 2 or more selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions. The carbonate refers to 1 or 2 or more selected from the group consisting of, for example, lithium carbonate, sodium carbonate, potassium carbonate, calcium carbonate, ammonium carbonate, magnesium carbonate, and barium carbonate.
Sulfate is a salt with sulfate ions as anions. The sulfate is preferably a salt of sulfate ion with 1 or more than 2 selected from the group consisting of alkali metal ion, alkaline earth metal ion and ammonium ion. The sulfate refers to 1 or 2 or more selected from the group consisting of, for example, lithium sulfate, sodium sulfate, potassium sulfate, calcium sulfate, ammonium sulfate, magnesium sulfate, and barium sulfate.
The chloride salt is a salt having chloride ions as anions. The chloride salt is preferably a salt of a chloride ion with 1 or 2 or more selected from the group consisting of alkali metal ions, alkaline earth metal ions, and ammonium ions. The chloride salt means 1 or 2 or more selected from the group consisting of, for example, lithium chloride, sodium chloride, potassium chloride, calcium chloride, ammonium chloride, magnesium chloride, and barium chloride.
The contact time or immersion time of the austenitic heat-resistant alloy in the molten salt is not particularly limited as long as it is 50 hours or more, and preferably 100 hours or more. The contact or impregnation time may be, for example, 3000 hours.
By making the base materialAn austenitic heat-resistant alloy having a Ni-Fe oxide having a spinel structure on the surface is brought into contact with or immersed in a molten salt to form NaFeO having a high growth rate and having a Ni-Fe oxide as a core 2 . Thereafter, in NaFeO 2 Is formed between the alloy and a base metal of an austenitic heat-resistant alloy (Fe, cr, ni) 3 O 4 . Thereafter, the alloy is further described in (Fe, cr, ni) 3 O 4 Cr is formed between the alloy and a base material of an austenitic heat-resistant alloy 2 O 3 . This can produce an austenitic heat-resistant alloy material.
Hereinafter, the present application will be described more specifically with reference to examples, but the present application is not limited to these examples.
Examples
Austenitic heat-resistant alloys having various chemical compositions of base materials and compositions of coating films were produced, and their molten salt corrosion resistance was examined.
[ investigation method ]
Ingots were produced by melting billets having alloy numbers 1 to 16 and having the chemical compositions shown in table 1. Referring to table 1, alloys having alloy numbers 1 to 10 are within the range of chemical composition of the base metal of the austenitic heat-resistant alloy of the present application. On the other hand, the alloys having alloy numbers 11 to 16 are outside the range of the chemical composition of the base metal of the austenitic heat-resistant alloy of the present application. Alloy No. 15 had a chemical composition equivalent to that of the known SUS 347H. Alloy number 16 has a chemical composition comparable to that of Alloy625, which is known.
TABLE 1
Figure BDA0002574815450000341
[ preparation procedure ]
The resulting ingot was heated to 1220 ℃, formed into a plate by hot forging, and cooled to room temperature. After cooling, the sheet was formed into a sheet having a thickness of 20mm by cutting the outer surface. Then, roll-rolling was performed at room temperature to form a plate having a thickness of 14 mm. Then, the plate was heated to 1200 ℃ and held for 15 minutes, and then water-cooled to produce an alloy plate.
[ pretreatment Process ]
The alloy plates of test numbers 1 to 11 were immersed in a pretreatment solution (8 mass% nitric acid, 3 mass% hydrofluoric acid, 2.5 mass% Fe ion concentration and 0.4 mass% Ni ion concentration) at 40 ℃ for 2 hours. The alloy plate of test No. 12 was immersed in a pretreatment solution (8 mass% nitric acid, 3 mass% hydrofluoric acid, 4.8 mass% Fe ion concentration, and 0.9 mass% Ni ion concentration) at 40 ℃ for 2 hours.
[ procedure for removing oxide skin ]
The alloy plates of test numbers 1 to 12 were taken out of the pretreatment solution and washed with water. Thereby, the scale attached to the surface of the alloy plate is removed.
[ step of Forming Ni-Fe oxide ]
The alloy sheets of test numbers 1 to 11 after washing were immersed in a Ni-Fe oxide forming solution (nitric acid 10 mass%, hydrofluoric acid 5 mass%, fe ion concentration 0 mass%, ni ion 0.2 mass%) at 30℃for 2 hours. The alloy plate of test No. 12 after washing was immersed in a 30 ℃ ni—fe oxide forming solution (nitric acid 10 mass%, hydrofluoric acid 5 mass%, fe ion concentration 2.4 mass%, ni ion concentration 0.5 mass%) for 2 hours. Through the above steps, austenitic heat-resistant alloys of test numbers 1 to 12 were produced.
The alloy sheets of test numbers 13 to 21 were subjected to 1 time of usual pickling. Specifically, hydrofluoric acid-nitric acid (nitric acid is 20% by mass+hydrofluoric acid is 1% by mass) is used for pickling for 10 minutes.
The surface of the base material of each test number alloy plate was analyzed for oxides formed thereon. Thereafter, corrosion tests in molten salt were performed on the alloy plates of each test number.
[ analysis of oxides ]
The oxides formed on the surface of the base material of each test number alloy plate were analyzed by the following method. From each test number of the alloy plate, a test piece having an oxide formed on the surface of the base material of the alloy plate was taken. The surface of the oxide was subjected to XPS to prepare a depth profile along the thickness direction of the oxide. Each element obtained by the depth profile was subjected to a state analysis and separated into an element in the form of an oxide and an element in the form of a metal. In the range from the surface of the oxide to the depth position when the detection intensity of O (oxygen) reaches half the detection intensity of oxygen at the surface of the oxide, it was confirmed that Ni, fe, and Cr were contained in the element existing in the form of the oxide. XPS was measured using the following conditions.
Device: XPS measuring device (ULVAC-PHI company, quantura SXM)
X-ray: mono-alkα ray (hν= 1486.6 eV), X-ray diameter: 100 μm phi
Neutralization gun: 1.0V, 20. Mu.A
Sputtering conditions: ar (Ar) + Acceleration voltage: 1kV and grating: 2X 2mm
Sputtering speed: 1.8 nm/min (SiO) 2 Converted value
Further, it was confirmed from the depth profile of test numbers 2 to 11 that peaks of Ni and Fe in the oxide form exist at a position shallower than the peak of Cr in the oxide form (a position near the surface of the oxide). Thus, it was confirmed that Cr oxide was present between the base material of each test number alloy plate and the Ni-Fe oxide. Points 2 before and after the peak position at which the concentration (at%) of Ni and Fe in the form of oxide reached the maximum were respectively designated as points a and B. The distance in the depth direction between the A-point and the B-point was measured and referred to as the thickness of the Ni-Fe oxide. Further, points 2, which are the concentration (at%) of Cr reaching half the maximum value before and after the peak position when the concentration (at%) of Cr existing in the form of oxide is the maximum, are designated as points C and D, respectively. The distance in the depth direction between the C-point and the D-point was measured and referred to as the thickness of Cr oxide. The results are shown in Table 2.
Next, raman spectroscopy analysis was performed on the oxide on the surface of the base material of each test-number alloy plate. Identification of 700-710 cm unique to spinel structured oxides from spectra obtained by Raman spectroscopy -1 Is a peak of (2). Thereby, austenite is identifiedNi-Fe oxide on the surface of the base material of the heat-resistant alloy. Identification of Cr from a spectrogram obtained by Raman Spectroscopy 2 O 3 550cm peculiar to -1 A nearby peak. Thus, cr oxide was identified.
Raman spectroscopy was performed using the following conditions.
Device: microscopic laser Raman spectrum measuring device manufactured by horiba of Kagaku Co., ltd. (LabRAM HR Evolution)
Measurement configuration: 180 ° backscatter configuration
Excitation wavelength: 532nm
Diffraction grating score: 600 strips/mm
ND filter: 25 percent of
Power: 2.3mW
Objective lens: 50 times of
[ molten salt Corrosion test ]
The resistance to molten salt corrosion of each test-numbered alloy sheet in molten salt was evaluated by the following test. Test pieces 1.5mm thick by 15mm wide by 25mm long were cut from the alloy plates of test numbers 1 to 12 after the Ni-Fe oxide forming step and the alloy plates of test numbers 13 to 21 after the pickling. After the test piece surface was ground with water-resistant grinding paper, degreasing and drying were performed and used for the test. The molten salt will mix NaNO in a 60:40 weight ratio 3 And KNO 3 And the resulting material was heated to 600 ℃. The test pieces were immersed in the molten salt at a test temperature of 600 ℃. The test time was 3000 hours.
The oxide formed on the surface of each test piece after the test was analyzed by XRD (X-ray diffraction) and EPMA (Electron Probe Micro Analyzer), and the structure of the oxide was identified by the above method. The results are shown in Table 2.
XRD was measured using the following conditions.
Device: RINT-2500 manufactured by Physics Co
X-ray vacuum tube: co ray
Wavelength of incident light:
Figure BDA0002574815450000371
scan range: 2θ=10 to 102°
Scanning step size: 0.02 degree
EPMA was measured under the following conditions.
Device: electron beam microscopic analyzer (JXA-8530F manufactured by Japanese electronics company)
Measurement magnification: 5000 times
Acceleration voltage: 15.0kV
Measurement method: element mapping
Measurement range: 18 μm in the thickness direction of the oxide x 18 μm in the direction perpendicular to the thickness of the oxide
In addition, scale formed on the surface was removed after the test. From the difference between the weight of the steel sheet before the test and the weight of the steel sheet after the test after the removal of the oxide scale, the corrosion loss (mg/cm) 2 ). The results are shown in Table 2.
[ test for measuring thickness of each oxide ]
EPMA analysis (element mapping) was performed on the cross section of the oxide on the surface of each test-number alloy plate after the molten salt corrosion test under the same conditions as those for the oxide analysis. By the above method, cr is measured from the concentration of each element 2 O 3 、(Fe、Cr、Ni) 3 O 4 And NaFeO 2 Is a thickness of (c). The results are shown in Table 2.
TABLE 2
Figure BDA0002574815450000391
[ test results ]
Table 2 shows the test results. The chemical composition of the base material of the alloy sheets of test numbers 1 to 11 was appropriate. The alloy sheets of test nos. 1 to 11 further had ni—fe oxide having a spinel structure on the surface of the base material. Therefore, the corrosion loss of the alloy sheets of test Nos. 1 to 11 was 8.0mg/cm 2 In the following, the molten salt corrosion resistance was excellent. Ni-Fe of alloy sheets of test Nos. 2, 4 to 6 and 8 to 11The oxide comprises NiFe 2 O 4 。
The alloy sheets of test nos. 2 to 11 further had Cr oxide between the base material and the ni—fe oxide. Therefore, the molten salt corrosion resistance was more excellent than the alloy sheet of test No. 1. Specifically, the corrosion loss of the alloy sheets of test Nos. 2 to 11 was 4.4mg/cm 2 The following is given. The Cr oxide of the alloy sheets of test Nos. 2 to 11 contains Cr 2 O 3 。
Further, the coating structures of the alloy sheets of test numbers 1 to 11 after the molten salt corrosion test were appropriate. Specifically, cr is provided on the surface of the base material of the alloy sheet after the molten salt corrosion test in order from the base material side 2 O 3 、(Fe、Cr、Ni) 3 O 4 And NaFeO 2 。
On the other hand, in the alloy sheet of test No. 12, although the chemical composition of the base material was proper, the Fe ion concentration and the Ni ion concentration in the pretreatment solution and the ni—fe oxide forming solution were too high. Therefore, the alloy sheet of test No. 12 does not have a ni—fe oxide having a spinel structure on the surface of the base material. As a result, the corrosion loss of the alloy sheet of test No. 12 was 10.2mg/cm 2 No excellent resistance to molten salt corrosion was exhibited.
The alloy sheets of test nos. 13 to 15 were subjected to ordinary pickling only 1 time, although the chemical composition of the base material was appropriate. Therefore, the alloy sheets of test nos. 13 to 15 did not have a ni—fe oxide having a spinel structure on the surface of the base material. As a result, the corrosion loss of the alloy sheets of test Nos. 13 to 15 exceeded 8.0mg/cm 2 No excellent resistance to molten salt corrosion was exhibited.
The base materials of the alloy sheets of test numbers 16 to 21 were not suitable in chemical composition. As a result, the corrosion loss of the alloy sheets of test Nos. 16 to 21 exceeded 8.0mg/cm 2 No excellent resistance to molten salt corrosion was exhibited.
The embodiments of the present application have been described above. However, the above embodiments are merely examples for implementing the present application. Therefore, the present application is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment within a range not departing from the gist thereof.

Claims (12)

1. An austenitic heat-resistant alloy comprising a base material and Ni-Fe oxide having a spinel structure on the surface of the base material, wherein Cr oxide is further provided between the base material and the Ni-Fe oxide,
the base material has the following chemical composition: in mass percent
C：0.030～0.120％、
Si：0.02～1.00％、
Mn：0.10～2.00％、
Cr:20.0% or more and less than 28.0%,
Ni: more than 35.0% and less than 50.0%,
W：4.0～10.0％、
Ti：0.01～0.30％、
Nb：0.01～1.00％、
sol.Al：0.0005～0.0400％、
B：0.0005～0.0100％、
Zr：0～0.1000％、
Ca：0～0.0500％、
REM：0～0.2000％、
Hf：0～0.2000％、
Pd：0～0.2000％、
P:0.040% or less,
S: less than 0.010 percent,
N: less than 0.020%,
O: less than 0.0050%,
Mo: less than 0.5 percent,
Co：0～0.80％、
Cu:0 to 0.50 percent
The balance: fe and impurities.
2. The austenitic heat-resistant alloy according to claim 1, wherein,
the Ni-Fe oxide having the spinel structure comprises NiFe 2 O 4 。
3. The austenitic heat-resistant alloy according to claim 1, wherein,
the Cr oxide comprises a material selected from the group consisting of Cr 2 O 3 And Cr (V) 2 O 3 ·yH 2 O, where y is an arbitrary rational number.
4. The austenitic heat-resistant alloy according to claim 2, wherein,
the Cr oxide comprises a material selected from the group consisting of Cr 2 O 3 And Cr (V) 2 O 3 ·yH 2 O, where y is an arbitrary rational number.
5. The austenitic heat-resistant alloy according to claim 1, wherein,
The chemical composition of the base material contains Zr in mass%: 0.0005 to 0.1000 percent.
6. The austenitic heat-resistant alloy according to claim 1, wherein,
the chemical composition of the base material contains Ca in mass%: 0.0005-0.0500%.
7. The austenitic heat-resistant alloy according to any of claims 1 to 6, wherein,
the chemical composition of the base material contains, in mass%, a material selected from the group consisting of REM:0.0005 to 0.2000 percent of Hf:0.0005 to 0.2000% and Pd:0.0005 to 0.2000% of at least 1 member of the group consisting of.
8. An austenitic heat-resistant alloy material comprising a base material and Cr on the surface of the base material 2 O 3 Said Cr 2 O 3 On (Fe, cr, ni) 3 O 4 And the (Fe, cr, ni) 3 O 4 NaFeO on 2 ，
The base material has the following chemical composition: in mass percent
C：0.030～0.120％、
Si：0.02～1.00％、
Mn：0.10～2.00％、
Cr:20.0% or more and less than 28.0%,
Ni: more than 35.0% and less than 50.0%,
W：4.0～10.0％、
Ti：0.01～0.30％、
Nb：0.01～1.00％、
sol.Al：0.0005～0.0400％、
B：0.0005～0.0100％、
Zr：0～0.1000％、
Ca：0～0.0500％、
REM：0～0.2000％、
Hf：0～0.2000％、
Pd：0～0.2000％、
P:0.040% or less,
S: less than 0.010 percent,
N: less than 0.020%,
O: less than 0.0050%,
Mo: less than 0.5 percent,
Co：0～0.80％、
Cu:0 to 0.50 percent
The balance: fe and impurities.
9. The austenitic heat-resistant alloy material according to claim 8, wherein,
the chemical composition of the base material contains Zr in mass%: 0.0005 to 0.1000 percent.
10. The austenitic heat-resistant alloy material according to claim 8, wherein,
the chemical composition of the base material contains Ca in mass%: 0.0005-0.0500%.
11. The austenitic heat-resistant alloy material according to any of claims 8 to 10, wherein,
the chemical composition of the base material contains, in mass%, a material selected from the group consisting of REM:0.0005 to 0.2000 percent of Hf:0.0005 to 0.2000% and Pd:0.0005 to 0.2000% of at least 1 member of the group consisting of.
12. A method for producing an austenitic heat-resistant alloy, comprising:
a step of preparing a blank having the chemical composition of claim 1;
a step of immersing the blank in a solution containing nitric acid and hydrofluoric acid, wherein the solution contains not more than 3.8 mass% of Fe ions and not more than 0.7 mass% of Ni ions, and performing pretreatment;
a step of removing the scale on the surface of the ingot by taking out the ingot from the solution; the method comprises the steps of,
immersing the ingot from which the oxide scale on the surface has been removed in a solution containing nitric acid, hydrofluoric acid, 1.2 mass% or less of Fe ions, 0.3 mass% or less of Ni ions, and a concentration of the nitric acid higher than that of the hydrofluoric acid, thereby forming a Ni-Fe oxide having a spinel structure on the surface of the ingot.
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